This invention relates to a geomagnetic direction sensor employing a magneto-resistance effect device.
In a color cathode ray tube, for example, the trajectory of an electron beam outgoing from an electron gun is occasionally bowed by terrestrial magnetism so that the electron beam is changed in its landing position on the fluorescent surface. In a high-definition cathode ray tube, changes in the landing (position deviation) tends to raise the problem of deteriorated color purity because of the small landing allowance.
For compensating these changes, a landing compensation coil is usually mounted on a cathode ray tube. An optimum current required for landing correction is caused to flow in the landing compensation coil, depending on the direction of the terrestrial magnetism, for controlling the trajectory of the electron beam for preventing mistaken landing.
Thus it is necessary to correctly detect the direction of the terrestrial magnetism. For this reason, a so-called geomagnetic direction sensor has been employed.
Such geomagnetic direction sensor is also employed as a goniometer in substitution for a conventional magnet goniometer (magnetic compass).
Among known types of the construction of the geomagnetic direction sensor, employed for a variety of usages, there are a so-called flux gate type and a magneto-resistive effect type.
The former construction of the geomagnetic direction sensor has a permalloy core 101 on which are wound a coil 102 for outputting electrical signals and an excitation coil 103, as shown in FIG. 1. The terrestrial magnetism is concentrated by the permalloy core 101 and thence transmitted to the coil 102 for outputting electrical signals.
With such flux gate type geomagnetic direction sensor, an ac bias magnetic field H.sub.B is generated by the excitation coil 103 in the permalloy core 101 and a pulsed electrical voltage generated on inversion of the bias magnetic field is detected as a signal. The voltage value of such pulsed electrical voltage is changed with the direction of terrestrial magnetism and hence may be utilized as a terrestrial magnetism sensor.
However, since the flux gate type geomagnetic direction sensor converts the terrestrial magnetism into an electrical signal by a coil, it is necessary to increase the number of windings of the coil for outputting electrical signals 102 for improving sensitivity or to increase the size of the permalloy core 101 for improving the effect of concentration. Thus it is difficult to reduce the size or production cost of the sensor.
The latter construction of the geomagnetic bearing sensor has a magneto-resistive sensor chip 110 having a magneto-resistive effect device 111 and an air-core coil 112 within which an MR sensor chip 110 is introduced, as shown in FIG. 2. An ac bias magnetic field H.sub.B is applied in a 45.degree. direction with respect to the MR sensor chip 110. FIG. 3 shows an equivalent circuit of the sensor. When the sensor is employed as the geomagnetic direction sensor, a set of the assemblies shown in FIG. 2 are used so that directions of the air-core coils are at right angles to each other.
Since the MR type geomagnetic direction sensor utilizes the magneto-resistive effect device, it is higher in sensitivity than the flux gate type geomagnetic direction sensor. However, since it senses geomagnetism only by the MR sensor chip 110, the direction of terrestrial magnetism on the order of 0.3 Gauss can be detected only insufficiently.
On the other hand, since the MR type geomagnetic direction sensor applies the bias magnetic field HB in a 45.degree.-direction with respect to the MR sensor chip 110, and an intentionally acute curve for MR characteristics is used for improving the detection sensitivity, the MR characteristics exhibit hysteresis, which has to be resolved by a complicated signal processing circuit. In addition, the direction detection accuracy is a low as .+-.10.degree..
Thus the conventional geomagnetic direction sensor leaves much to be desired in detection sensitivity, while it can be reduced in size and production cost only with considerable difficulties.